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A century on: how the work of GCHQ's predecessors contributed to the US entering World War I

The 'Zimmermann Telegram' of 1917 demonstrates the need to balance making use of intelligence and protecting its source, which is still relevant today.

One hundred years ago in January 1917, GCHQ's predecessors in Room 40 of the Admiralty produced an intelligence report which contributed to the United States, still neutral at that point, entering World War I on the Allied side.

While the war on the Western Front had reached stalemate, the Allied blockade of Germany was beginning to bite hard. There were increasing shortages of food and of the raw materials need for Germany to keep on fighting. By contrast, between the Royal Navy's patrol activity in the Atlantic, and the restrictions placed on German submarines about which ships they could and couldn't attack, the Allies were receiving supplies from the US which enabled them to continue to deny Germany the opportunity to break through in the West.

Early in January 1917 the German leadership decided to institute a new campaign of unrestricted submarine warfare. Although they recognised the danger that the sinking of American ships carrying supplies to the UK might bring the United States into the war, they were confident that the UK would be brought to the point of surrender long before any US troops could reach Europe.

The German Foreign Ministry decided to play its part by seeking to neutralise any potential US response. The German Foreign Minister, Arthur Zimmermann, proposed an alliance with Mexico: if the US joined the Allied side in the war and Mexico would ally itself with Germany, it would receive Texas, New Mexico and Arizona as prizes after the defeat of the Allies.

At the start of the war, the UK had cut the German telegraphic cables which ran through the English Channel, removing the German capability to communicate directly with North and South America, and forcing them to find other ways to reach their diplomatic missions there. One method, nicknamed the "Swedish Roundabout", was to have the Swedish Foreign Ministry send their messages for them, disguised as Swedish diplomatic messages. This was supplemented in 1916 with the offer by US President Wilson to allow the Germans to send diplomatic telegrams through US diplomatic channels if this would help advance Wilson's efforts to bring peace to Europe. In what seems an act of outrageous cheek, the German Foreign Ministry used this channel to send its offer to Mexico via its Embassy in Washington on 16 January 1917.

All telegraphic traffic between neutral countries in Europe and North America had to transit the UK, and this meant that enciphered messages would be brought to the attention of Room 40, or their War Office colleagues in MI1(b).

The message was read on 17 January by a team including Nigel de Grey, and its importance was quickly realised. However, using the intelligence it contained - that Germany was offering an alliance and the promise of US territory to Mexico - was complicated. How could the material be shown to the US authorities without revealing that it had been obtained by reading American diplomatic telegrams? And how could the information be made public without revealing to the Germans that their diplomatic codes were being read?

Admiral Hall, the Director of Naval Intelligence, and therefore in charge of Room 40, got round the first part of the problem by acquiring in Mexico City a copy of the message between the German Embassies in Washington and Mexico City, and this is what was shown to the US Embassy in London. It was leaked to the US press and was front page in most US newspapers on 1 March 1917. The US Secretary of State, Robert Lansing, dropped deep hints about the acquisition of the telegram by a spy, and the German authorities in Berlin eventually accepted that this was probably the case. In any event, Zimmermann publicly announced that the telegram was authentic, and in April the United States declared war on Germany.

Did the Zimmermann Telegram bring the United States into the war? Not by itself: America was moving away from the isolationism which had characterised it in the first years of the war; and the deliberate sinking of US merchant ships would undoubtedly have changed the American mood. Yet publication of the telegram contributed to changing American perceptions of Germany: it was suddenly much harder to say that one side was as bad as the other when an offer had been made to Mexico to annex three states.

Most importantly, to the cryptanalysts in London at least, the Germans never realised that their diplomatic codes were readable, or, more generally, that the UK had been involved at all.